This invention relates generally to injection molding machines. More particularly, this invention concerns vented injection molding machines having a two-stage screw therein or extruder screws wherein the first stage surges.
An injection molding machine typically receives synthetic resinous material as a particulate feedstock, heats, masticates and plasticates the material to a moldable consistency, and then forces the plasticated material into a mold. In the past, readily volatilized substances or moisture present in a particulate synthetic resinous material feedstock for an injection molding machine frequently led to defects in the molded product produced by the machine. Many defects are attributed to the evolution of gaseous pockets containing water vapor or volatilized substances. For example, defects such as polymer degradation and diminished physical material properties in a molded product have been attributed to such gaseous pockets. Moreover, the gaseous pockets cause surface defects in the molded product, such as loss of gloss and voids. These latter defects are sometimes merely cosmetic but do, on occasion, substantially affect physical properties of the molded product. Moisture and volatilizable substances must therefore be eliminated from synthetic resinous material during the molding thereof.
One solution for avoiding moisture in feedstock requires a separate drying apparatus to predry hygroscopic synthetic resinous material and to remove the moisture therefrom before supplying the material to an injection molding machine. However, a separate drying apparatus involves an additional capital expense which reduces the profitability of the injection molding apparatus.
To accommodate a feedstock having readily volatilized substances therein, an injection molding machine should be provided with a vent to allow gaseous volatiles to escape after the feedstock has been plasticated and masticated to a molding consistency. The known prior art vented molding machines, however, are subject to bleed of plasticized material into the vent opening with the concomitant problem of possible blockage of the vent opening.
When dealing with feedstock having fine particle size, such as a powdered feedstock, a vent opening is also necessary in a molding machine to eliminate air occlusions which may occur from air pockets fed along with the feed material into the plasticating screw of the molding machine.
Examples of particular synthetic resinous materials for which venting is desirable during injection molding are as follows: polyesters, polycarbonates, nylons, acrylics, ABS, styrenic polymers, acetal polymers, polyphenylene oxides, and barrier resins (such as "Barex", and "Lopac").
In fashioning articles of indeterminate length from synthetic resinous materials, vented extruders allow removal of volatiles as well as moisture from feedstock. Unlike injection molding machines, however, extruders operate with essentially steady material flow into the machine, essentially steady or continuous flow of product material out of the machine, and essentially steady flow of material through a vent section of the extruder machine. Because vented extruders operate at steady state, only small fluctuations in flow conditions inside the barrel at a vent opening typically occur.
A typical injection molding machine, by contrast, operates in a three-part cycle, a plasticating portion during which feedstock is prepared to a molding consistency, an injection portion during which the plasticated material is injected into a mold cavity, and a holding portion during which material in the mold is permitted to solidify. The cyclical operation of the molding machine causes unsteady flow conditions to prevail inside the molding machine, which in turn contribute to the problem described above wherein plasticized material bleeds into the vent opening.
As a result of their cyclical operation, vented screws for injection molding have to accommodate a large surge of plastic through the vent section without pushing plasticized material out of the vent. The surge occurs because, unlike the continuous extrusion process, in injection molding, the screw stops rotating during the holding portion. During this time, plastic continues to melt in the hot barrel, particularly in the first pumping section as shown in the prior art machine in FIGS. 1 and 2. When the screw resumes rotating, the pool of melted material is quickly and easily conveyed along the screw, thereby forming a surge which causes the above noted vent bleed problem.
Moreover, during the injection portion of a molding cycle, a plasticating screw is typically impulsively stroked forward through a distance of several inches to inject a charge of plasticated material into a mold cavity. During this injection portion, pressures in the plasticated material on the order of 20,000 psi have been known to exist and to aggravate the vent bleed problem.
Particularly undesirably, the plasticated material may solidify in the vent opening and thereby block the vent opening. Thereafter, the subsequent release of volatilized material from the vent opening is severely inhibited, if not actually prevented.
Various injection molding machines have been proposed in the prior art to overcome problems of the type described above. However, each of the prior art machines is objectionable by virtue of having one or more of the following disadvantages: narrow range critical operation; low plasticating capacity; and extensive modifications of a conventional mechanical or hydraulic operating system.
One particular example is the injection molding machine disclosed in U.S. Pat. No. 4,074,362, which is assigned to the assignee of the present invention and is hereby incorporated herein by reference. U.S. Pat. No. 4,074,362 discloses a metering blister 84 disposed between the first and second stages of the screw. In such a design, at the onset of the plastication portion of the molding cycle, metering blister 84 "meters" the flow of plasticated material exiting the first stage. However the blister disclosed by U.S. Pat. No. 4,074,362 only restricts flow. It does not suppress surges of material but merely limits the maximum flow thereacross. Indeed, a design such as that shown in U.S. Pat. No. 4,074,362 may actually aggravate a surge problem. That is, excess plasticated material builds up along the first stage of the screw during the surge because the metering blister prevents the flow of such material thereover. Adjustments must be made for this excess material, such as slowing the screw rpm, which in turn leads to the above described problem of lowering the plasticating capacity.
Thus, a need continues to exist for an effective injection molding machine having a vent that permits volatile materials and water vapor to be released from a plasticized material during an injection molding machine cycle without material bleeding into and clogging the vent. In particular, a need continues to exist for an effective way of addressing the surge induced vent bleed problem without reducing the plasticating capacity of the machine.